Cavity resonator tuned by variable recessing, instead of variable projecting, tuning screw



Feb. 16, 1965 K. D. POWELL CAVITY RESONATOR TUNED BY VARIABLE RECESSING. INSTEAD OF VARIABLE PROJECTING, TUNING SCREW Filed March 1, 1961 Fig.2.

ECTIONAL OUPLER DIR DISCRIMINATOR CAVITY INVENTOR Kenneth D. Powell ATTORNEY KLYSTRON DIFFERENTIAL AMPLIFIER Fig.6.

Fig.5.

wrmesses:

BMMQRQ- WJ United States Patent CAVITY RESUNATGR TUNED BY VARIABLE RECESSING, INSTEAD OF VARIABLE PRO- .IECTHNG, TUNING SiZREW Kenneth 1). Powell, Horseheads,N.Y., assignor to Westinghouse Electric (Corporation, East Pittsburgh, Pa, a corporation of Pennsylvania Filed Mar. 1, 1961, Ser. No. 92,540 6 Claims. (Cl. 333-83) This invention relates to micro-wave cavity resonators and more particularly to those resonators designed to 0perate with at least two degenerate modes. This type of cavity is more commonly referred to in the artas a dualmode cavity.

The particular device described herein is a fixed frequency dual-mode transmission type cavity. The cavity is designed to operate at a fixed frequency as a frequency determining device. The cavity is designed to provide a smooth, balanced discriminator curve when operated with matched detectors. 7

A typical circuit employing the cavity resonator as a frequency sensing element is illustrated in FIG. 6. In this circuit the cavity resonator provides a zero output signal at the desired operating frequency of the klystron. Each of the outputs from the cavity resonator sees essentially a separate cavity. This is because of the dual-mode cavity. The dual-mode cavity provides, in effect, two electrical cavities while in reality only one physical cavity is provided. The result is that excellent symmetry between the outputs is obtained from the cavity resonator. In the particular circuit shown in FIG. 6, the cavity resonator is utilized for stabilizing the operating frequency of the klystron tube. A portion of the output from the klystron is coupled by means of the directional coupler to the discriminator cavity. The mode coupled into the discriminator cavity is split into a degenerate form. This may be accomplished by deforming a cylinder having a circular cross section to provide an elliptical cross-section. The field configuration of an elliptical cavity can be simulated by placing a perturbing element in one side of a circular cavity and thereby decrease the cross-section in one plane. This plane may be referred to as a minor axis of the tube while the other axis perpendicular thereto would be considered the major axis so as to thereby simulate an elliptical cavity. The two modes of oscillation taking place in the cavity resonator are preferably separated as to frequency by a relatively small amount. In the particular application in which the operating frequency is of about 35,000 megacycles, the resonate frequencies of the .two modes may be separated by about ten megacycles.

Energy taken by the two outputs from each mode of oscillation is coupled into a differential amplifier. By proper adjustment of the dilierential amplifier, it can be arranged that for a predetermined desired frequency of the klystron between the two resonate frequencies of the two modes of oscillation of the resonator, no correction voltage will be applied to the reflector of the reflex klystron. If the frequency of the klystron varies from the desired value, the

amplitude of one mode of oscillation will increase while that of the other mode of oscillation will decrease. Accordingly, the reflector of the klystron will have its voltage vary in a positive or negative direction from its standard voltage tending to retune the oscillator to restore its frequency toward the predetermined value. Thus, any deviation of the klystron oscillator from its desired operating frequency results in a correction voltage applied to the reflector of the klystron. In this way, the ldystron may be maintained at a fixed frequency.

As indicated above, the field configuration of an elliptical cavity can be simulated by placing a perturbing element such as a plunger or a screw which projects into a cylin- 3,l 5 0,152.10 Patented Feb. 1 6, 1065 megacycles to simulate the elliptical field configuration.

At very high frequencies such as 35,000 megacycles a screw protruding into the cavity is very lossy and a low Q results. One reason for this problem is that at higher frequencies the cavity becomes quite small. For example, the dimensions of a cavity at 35,000 megacycles are such that the diameter of the cavity is .336 inch and the height of the cavity is .414 inch. With these dimensions of'the cavity, the tuning screws used reached a practical lower limit in size. The ratio of the screw to cavity surface becomes greater and greater and consequently the amount of current interrupted by the high resistance joint becomes larger.

It is, accordingly, an object of this invention to provide an improved means of tuning a cavity resonator by means of a wave guide beyond cut-off to obtain an improved cavity resonator.

It is another object to provide an improved dual-mode cavity resonator in which the tuning between the two degenerate modes is accomplished by means of a wave guide beyond cut-off to provide a high Q cavity.

It is' another object of this invention to provide tuning of the degenerate modes within a dual mode cavity so that tuning is achieved at a low current point within a wave guide beyond cut-01f provided in the wall of the cavity resonator.

These and other objects of this invention will be apparent from the following description taken in accordance with the accompanying drawings, throughout which like reference characters indicate like parts, and which drawings form a part of this application and in which:

FIGURE 1 is a front view of a cavity resonator in ac,- cordance with the teachings of this invention;

FIG. 2 is a top view of the cavity resonator illustrated in FIG. 1;

FIG. 3 is a bottom view of the cavity resonator illustrated in FIG. 1;

FIG. 4 is a sectional View taken along the line IV of FIG. 3;

FIG. 5 is an enlarged view of the wave guide beyond cut-off tuning mechanism illustrating the field configuration therein; and

FIG. 6 is a schematic illustration of a particular application of the cavity resonator illustrated herein.

Referring nowto the drawings, there is shown a rectangular body ltlhaving a cylindrical opening 12 provided therein. The cavity bodyitl is of a suitable material such as Invar having a silver plating provided thereon for conductive reasons. It is also possible to fabricate the cavity body 10 of a conductive material such as copper. The cylindrical opening 12 in the cavity body 10 is closed at the upper end, as illustrated in FIG. 1, by an end plug 14. The end plug 14 has a projecting portion 16 which projects within the cylindrical opening '12 and forms and defines one end of a cavity resonator 20. The end plug 14 is brazed tothe cavity body 10 by well known brazing techniques. The other end of the cylindrical opening 12 is closed by the end cover 22 whichis also brazed to the cavity body to provide a vacuum tight seal. The end cover 22 defines the other end of the cavity resonator 20. The end cover 22 is provided with a rectangular wave guide 24 having a window of a suitable material such as mica sealed therein to provide means of directing the input energy into the cavity resonator 20. The two outputs from the cavity resonator 20 are derived by means of wave guides 26 and 28, as illustrated in FIG. 3, coupled through the cavity body It with suitable windows of a material such as mica also provided therein to couple energy from the cavity 20. As is indicated in FIG. 3, the one output wave guide 26 lies along the axis X of FIG. 3 and extends to the right of the cavity resonator 20 while the output wave guide 28 is shown along the axis Y in FIG. 3 and extends downwardly from the cavity resonator 20.

The center frequency tuning of the cavity resonator 20 is achieved by perturbing the field symmetrically with respect to the degenerate modes. A convenient location for this tuning element is at the center of the end cover 14. The tuning may be accomplished by use of a simple screw protruding into the cavity resonator or also by the use of a wave guide beyond cut-off. In the specific application shown and illustrated in the FIGS. 2 and 4, a capacitively shorted nose tuning arrangement 29 is utilized. The structure 29 consists of a screw 30 having a nose 32 so that the screw is threaded into an opening 34 in the end cover 14. The nose 32 is also of Invar having a silver coating thereon and projects through an iris 36 into the cavity 20. The diameter of the nose 34 is .025 inch and the diameter of the iris is .040 inch. It was found that such a structure provided a 25 megacycle tuning with negligible loss in Q.

In order to achieve a symmetrical discriminator, it is necessary to be able to change the coupling on one of the degenerate modes to the external load. In the applicants device, this is provided by placing asymmetries within the wall of the cavity. In the specific embodiment shown, a tuning structure 40 similar to the tuning structure 29 utilized for center tuning is provided in the end plug 14 and is off-set from the center of the end cover as indicated in FIG. 2. The tuning structure 40 in FIG. 2 indicates a capacitively shorted nose tuning arrangement similar to the tuning structure 29.

The output coupling and the wave guides beyond cutoff are located at voltage maximum regions axially, and diametrically opposite. The specific structure utilizes a TE mode.

As indicated previously the system employed for tuning the degenerate modes is a wave guide beyond cutoff. In a wave guide beyond cutoff, there is attenuation only and no phase shift. The attenuation is not a dissipative attenuation as that due to resistance in a transmission system with propagating waves. It is purely reactive attenuation, analogous to that in a filter section made of reactive elements. The energy is not lost but is reflected back to the source so that the guide acts as a pure reactance to the source. It is found that the wave guide beyond cutoff adds a small amount of cross-section in one plane and no losses. The fields and currents in a wave guide beyond cutoff fall off at the rate of an exponential function. Tuning can be achieved at a low current point in the wave guide. A screw contact may be utilized with little effect to the Q because the high resistance joint is now at a low current point as is indicated in FIG. 5. The cross-hatched region in FIG. indicates the added volume effective to the fields within the cavity 20.

In the specific embodiment shown in the drawing, an opening 50 is provided in the cavity body diametrically opposite the output wave guide opening 28 and a corresponding wave guide beyond cutoff opening 60 is provided in the wall of the cavity body 10 diametrically opposite the other output wave guide opening 26. In the specific embodiment shown, a screw 52 is provided within the threaded opening 50 as indicated in FIGS. 3 and 4 for tuning the degenerate modes. The other wave guide 60 is utilized for exhaust. An exhaust tubulation 62 is shown in FIGS. 1 and 3 and it is utilized in evacuating the cavity 20 and sealing off the cavity 20 to provide a vacuum enclosure. However, if one should obtain a noncylindrical cavity such that tuning is required in the opposite direction then, of course, the other wave guide 60 may be provided with a screw and utilized for tuning All the cavity resonator 20. It is, therefore, obvious that by adjusting the position of the screw 52 within the wave guide beyond cutoff one may obtain the required asymmetry to tune the degenerate modes with very little loss of Q. The diameter of the wave guide beyond cutoff is .086 inch. It should be noted that with this diameter the opening 50 provides a wave guide beyond cutoff and therefore there is an effective volume provided within the wave guide due to the field penetrating the wave guide beyond cutoff. If the diameter of the opening were such as to provide a wave guide far beyond cutoff, then the attenuation per unit length is very high and the guide would not be useful as a tuning element. To be useful, the diameter of the wave guide beyond cutoff should not be smaller than A of the resonant wave length of the cavity. Thus, by adjusting the position of the screw 52 within the wave guide beyond cutoff 50, one is able to vary the difference frequency between the two modes within the cavity 20. In normal practice the tuning screws utilized for center tuning, balancing and degenerate mode tuning are locked into position and soldered prior to being shipped from the factory.

While the present invention has been shown in only one form, it will be obvious to those skilled in the art that it is not so limited, but is susceptible to various changes and modifications without departing from the spirit and scope thereof.

I claim as my invention:

1. A cavity resonator comprising a member having a hollow cylindrical opening therein, a top closure means for said hollow opening, a bottom closure means for said hollow opening to define a cavity adapted to be excited simultaneously in two different modes, means for coupling energy into said cavity resonator of a substantially single frequency to excite two modes of oscillation within said cavity, means for selectively tuning said two modes of oscillation from which energy is derived through the walls of said cavity by means of two output means, said tuning means comprising an opening provided in the side wall of said cavity of a diameter to provide a wave guide beyond cutoff and an adjustable member provided within said opening for varying the length of the opening connected to said cavity.

2. A cavity resonator comprising a plurality of walls defining a cavity, means for coupling energy of a substantially single frequency to excite two modes of oscillation within said cavity, means for selectively tuning the frequency difference between said two modes of oscillation from which energy is derived through the walls of said cavity by means of two output means, said tuning means comprising an opening provided in a wall of said cavity of a diameter to provide a wave guide beyond cutoff and an adjustable member provided within said opening for varying the length of the opening connected to said cavity.

3. A cavity resonator comprising a member having a hollow cylindrical opening therein, a top closure means for said hollow opening, a bottom closure means for said hollow opening, means for coupling energy of substantially single frequency to said cavity resonator to excite two modes of oscillation within the cavity defined by the walls of said hollow opening in said top and bottom closure means, means for selectively tuning the frequency difference between said two modes of oscillation from which energy is derived through the walls of said cavity by means of two output means, said tuning means comprising an opening provided in the side wall of said cavity of a diameter to provide a wave guide beyond cutoff and an adjustable member provided within said opening for varying the length of the opening connected to said cavity.

4. A resonant cavity formed of upper and lower faces and side walls, a cylindrical opening in said walls having a diameter chosen to form a wave guide having a cutoff wave length at the operating wave length of said 53 cavity to provide an additional volume within said cavity, a tuning probe extending into said opening and confined therein, said probe being movable within said opening to vary the field within said opening and thereby the operating wave length of said cavity.

5. A resonant cavity formed of upper and lower faces and side walls, input means provided in one of said faces for introducing energy into said cavity, a first output means provided in the side wall of said cavity, a first opening provided in said side wall diametrically opposite said first output means, the dimensions of said first opening chosen to form a Wave guide having a cutofi wave length less than the operating Wave length of said cavity, a second output means positioned in said. side wall intermediate said first output means and said first opening, a second opening provided in the side wall diametrically opposite said second output means, "the dimensions of said second opening chosen to form a wave guide beyond cutofi at the operating wave length of said cavity, a tuning probe extending into said second opening and confined therein, said probe being movable Within said opening to vary the operating frequency of said cavity.

6. A resonant cavity formed of ,upper and lower faces and of side walls, a Wave guide beyond cutoff provided in said side walls to provide an additional volume within said cavity, a tuning member extending into said wave guide and confined therein, said tuning member being movable within said Wave guide to vary the field within said Wave guide connected tosaid cavity to thereby vary the operating Wavelength of said cavity.

References Cited in the file of this patent UNITED STATES PATENTS 2,423,383 Herschberger July 1, 1947 2,694,795 Pureka Nov. 16, 1954 2,854,532 Robson Sept. 30, 1958 OTHER REFERENCES Southworth: Principles and Applications of Waveguide Transmission, Van Nostrand, New York, 1950, pages 258 and 261 relied on.

Microwave Transmission Circuits, volume 9 of the Radiation Lab. Series, McGraw-Hill, 1948, pages 336, 337 and 502 cited. 

6. A RESONANT CAVITY FORMED OF UPPER AND LOWER FACES AND OF SIDE WALLS, A WAVE GUIDE BEYOND CUTOFF PROVIDED IN SAID SIDE WALLS TO PROVIDE AN ADDITIONAL VOLUME WITHIN SAID CAVITY, A TUNING MEMBER EXTENDING INTO SAID WAVE GUIDE AND CONFINED THEREIN, SAID TUNING MEMBER BEING MOVABLE WITHIN SAID WAVE GUIDE TO VARY THE FIELD WITHIN SAID WAVE GUIDE CONNECTED TO SAID CAVITY TO THEREBY VARY THE OPERATING WAVELENGTH OF SAID CAVITY. 